1. Field of the Invention
The present invention relates to protected optical disks which contain inside a housing made in several parts at least one layer for storing data written and read optically.
2. Description of the Prior Art
For the data storage layer use may be made of heat-deformable structures producing markings in relief by surface marking using finely focussed laser light. The relief markings may consist in an alteration by ablation or in a rounded protuberance. Equally envisagable is a marking without relief in light-sensitive layers based on silver-containing or diazotype materials. It is also envisagable to use magnetic or metallic layers in which the marking consists in a phase change following thermo-optical writing.
All these layers share considerable vulnerability to external agents and require appropriate protection achieved by enclosing them in a rigid housing through which light can pass.
In the case of optical disks, the protective housing generally acts as a support for the thin layer which it has to protect. As the housing has a radial dimension which is significantly greater than its other dimensions, the parts from which it is made up must be assembled in such a way as to avoid any loss of flatness of the carrier layer. In order to write or read data the sensitive layer must be scanned with an extremely small light spot which is obtained in the immediate vicinity of the focus of a light beam subtending a large angle. To achieve the optical exposure of a substantially plane layer, the optical disk is centered on a shaft rotated about an axis perpendicular to the layer and the write or read beam is moved radially, in order to trace out a track. Proper track following entails the point of convergence of the radiation being coincident with the surface of the layer, whence the necessity for a focussing control system. Defects in respect of the flatness of the sensitive layer must be taken into account by the focussing control system and are more often than not directly associated with distortion of the housing constituting the optical disk since the layer tends to adopt the shape of its protective casing. When an optical disk housing is made by assembling two superposed circular flanges, with a circular weld joining them, the overall structure is subject to residual mechanical stresses due to shrinkage of the material melted during welding. These stresses may compromise the flatness of the flange carrying the sensitive layer to the point that warping of the optical disk and/or buckling of its structure make it impossible to achieve focussing because of undulations affecting the circular perimeter during scanning. In theory, the binding effect of a weld on a structure with a symmetry of revolution should result in purely radial curvature of the layer support such that each circular track element remains situated in a plane orthogonal to the rotation axis. Moderate curvature of this kind, producing a bell shape, may be taken into account by the focussing control system.
In practice, this circumferential flatness leaves something to be desired even if care is taken to use parts having all the required elastic and geometrical characteristics, since the bell-shaped deformation may be of greater amplitude than catered for by the reading system.
In U.S. Pat. No. 4,564,932 there is described a method of manufacturing a protected optical disk which consists in welding together the thermoplastic material parts constituting the housing. The heat necessary for welding is applied in the form of ultrasonic vibration causing melting of the thermoplastic material in a number of concentric annular areas of restricted width in the radial direction, in order to obtain a joint comprising multiple welds. The improvement in terms of the flatness qualities of a disk obtained by this method arises from the fact that a multiple weld joint can offer the same resistance to separation of the parts as a single weld joint but with reduced circumferential forces by virtue of the fact that these forces are proportional to the cross-section of the molten material. Thus a double weld joint involves a quantity of molten material half that employed to form a single weld twice as wide and twice as deep. Ultrasonic welding entails a preparatory stage which consists in providing on one of the surfaces to be joined a sharp-edged projection which serves to concentrate on a circular line the thermal focus which gives rise to the weld. The flatenning of this projection during welding is a parameter which is difficult to control, which explains a lack of circumferential regularity of the weld, even if its location is better defined radially than in the absence of a projection of this kind. Furthermore, the excess material constituting the projection is squeezed by the flat surfaces on either side of it, leaving a narrow interstice which promotes the concentration of mechanical stresses. It is thus seen that the preparation of the surfaces to be joined by the preliminary formation of circumferential projections means that the final shape of the weld and the residual stresses which it produces are not totally controlled.
With a view to alleviating this disadvantage, the invention proposes that the surfaces to be joined should be preformed with a recessed profile contributing to improved delimitation of the weld and to reducing its circumferential stiffness by virtue of radial separation which renders shrinkage phenomena less crucial with regard to the optical disk. This more suitable shape is obtained by preforming the surfaces to the joined with two circular grooves one on either sie of the sharp-edged projection which produces the weld on local melting by ultrasonic means.